Particle surface roughness is the other ones affecting variables on flotation  

Wenzel suggested that the geometry of the surface had a greater effect on the static contact angle than did the chemistry!..

The wettability of real surfaces is subject to many factors, both physical and chemical. For the flotation process, the effect of four main factors; surface roughness, heterogeneity, particle shape, and particle size, on contact angle measurements are of importance (Chau et al., 2009).

T.T. Chau, W.J. Bruckard, P.T.L. Koh, A.V. Nguyen, A review of factors that affect contact angle and implications for flotation practice, Advances in Colloid and Interface Science 150 (2009) 106–115.

Figure: Static contact angle ( advancing angle and the receding) of water on wax substrates, as a function of the substrate roughness (Johnson, Dettre, 1964).

For hydrophobic surfaces the data of Johnson, Dettre, (1964), reported in the Figure above, show the effects of advancing θa (maximum static angle) and receding θr (minimumstatic angle) contact angles of water on surfaces of wax of variable roughness.

As the roughness increases, small variations in the angles, together with significant increases in hysteresis, are observed. Then, both angles suddenly increase as the hysteresis diminished. It was concluded that texturing a solid does not only modify the hysteresis but it can also affect dramatically the value of the mean contact angle itself.

Johnson RE, Dettre RH. Adv Chem Ser 1964;43:112–35.

Wotruba et al. [1991] reported contradiction results from flotation tests carried out on zircon samples before and after attritioning for 48h in a planetary mill. It was noted that the particle shape remained unchanged by the attritioning and only the particle surfaces were roughened. It was shown that the freshly roughened material floated in total far better than the original material.

Wotruba H, Hoberg H, Schneider FU. XVIIth Int Miner ProcessCong IV 1991:83–95.

Successful particle–bubble attachment involves three steps, viz., 1) thinning of the intervening liquid film to a (critical) thickness where the film ruptures, 2) rupture of the intervening liquid film and formation of a three-phase contact (tpc) nucleus (a hole of a critical wetting radius) and 3) expansion of tpc line from the critical radius to form a stable wetting perimeter (Chau et al., 2009).

T.T. Chau, W.J. Bruckard, P.T.L. Koh, A.V. Nguyen, A review of factors that affect contact angle and implications for flotation practice, Advances in Colloid and Interface Science 150 (2009) 106–115.